SimpleEdit allows you to create your own and to edit existing themes. These themes can be easily created and edited with SimpleEdit itself or you can use any other Plist Editor. The themes are stored in a sub folder of its 'Application Support Folder'.

This can be found in the Mac App Store version here: ~/Library/Containers/com.myownapp.SimpleEdit/Data/Library/Application\Support/SimpleEdit/Themes

And in the Non-MAS version here:~/Library/Application\Support/SimpleEdit/Themes

These themes are 'loaded' every time you start SimpleEdit or when you create a new document. A theme is nothing else than a dictionary with objects and keys - much like JSON. It is based on RGBA colors most of us already know from CSS: color: rgba(255,255,255,1.0);

You can create as many themes as you like to and apply them in the Preferences.